1. Field of the Invention
The present invention relates to a semiconductor device and its manufacturing method which can be applied to a quantum interference device or a one-dimensional channel field effect transistor (FET).
2. Description of the Prior Art
In recent years, researches and developments of quantum interference devices using interference effects of electron waves have actively been performed. As one kind of quantum interference device, a quantum interference transistor using Aharonov-Bohm effect (hereinafter, referred to as an AB effect transistor) has been known. In the AB effect transistor, the transistor action is executed by using the interference effect of electron waves passing through a multichannel.
FIGS. 1 and 2 show examples of conventional AB effect transistors using a planar multichannel. In the AB effect transistor shown in FIG. 1, a multichannel 101 is formed by a bulk doped semiconductor or a modulation-doped semiconductor. Although one multichannel 101 is formed at positions near the source and drain, it bifurcates into two channels which form a ring shape as a whole at a location between the source and the drain. In this case, an electron wave which enters from one end on the source side (for instance, from the left edge in the diagram) into the multichannel 101 is separated into an electron wave passing through the path A and an electron wave passing through the path B in the ring-shaped portion. Thereafter, they are again joined into one electron wave. Upon joining, an interference between the electron waves occurs. By applying a magnetic field so as to penetrate the ring shaped portion of the multichannel 101 and controlling the phase difference between the electron waves passing through the paths A and B by the magnetic field, the transistor action is executed. On the other hand, in the AB effect transistor shown in FIG. 2, the operation principle is fundamentally the same as that of the AB effect transistor shown in FIG. 1 although it differs therefrom with respect to points that the multichannel 101 has a rectangular shape as a whole at a position between the source and the drain and that the phase difference between the electron waves passing through the paths A and B is controlled by a voltage which is applied between a pair of gate electrodes G.sub.1 ' and G.sub.2 ' arranged adjacently at the outside positions of the multichannel 101.
On the other hand, FIG. 3 shows another conventional AB effect transistor using a vertical type (depth direction type) multichannel (Technical Digest of IEDM 1986, P.76, 1986). As shown in FIG. 3, in such an AB effect transistor, the multichannel 101 is formed by double quantum wells comprising modulation-doped semiconductors. Reference numeral 102 denotes a barrier layer. The multichannel 101 bifurcates into two channels on and under the barrier layer 102 at positions between the source and the drain. Even in the case of the AB effect transistor shown in FIG. 3, similarly to the case of the AB effect transistors shown in FIGS. 1 and 2, the phase difference between the electron wave passing through the path A and the electron wave passing through the path B is controlled by a gate voltage which is applied to the gate electrode (not shown) formed on the path A, thereby allowing the transistor action to be executed.
In the conventional AB effect transistors shown in FIGS. 1 and 2, the larger the aspect ratio (=diameter/width) of the multichannel 101 is, the higher the interference effect of the electron waves passing through the paths A and B is and better transistor characteristics can be obtained. Therefore, it is desirable to make the channel finer. However, the impurity concentration in the bulk doped semiconductor is not so high. In the case of using the modulation doping, the concentration of two-dimensional electron gas (hereinafter, referred to as 2DEG) which is formed at the heterojunction interface is not so high, either. Therefore, the width of the depletion layer which is formed on the side surface of the multichannel 101 is fairly large. Thus, although the effective channel width of the multichannel 101 must be extremely narrow, it is actually difficult to make the channel finer.
On the other hand, in the conventional AB effect transistor shown in FIG. 3, a high interference effect of the electron waves is not yet realized. In addition, just like in the case of the AB effect transistors shown in FIGS. 1 and 2, it is difficult to make the channel finer because of the depletion layers which are formed on both side surfaces of the multichannel 101. As a result, the aspect ratio of the channel becomes small.
Further, the AB effect transistors shown in FIGS. 1, 2 and 3 can be operated only at a very low temperature and it is difficult to operate them at a higher temperature.
On the other hand, in recent years, attention has been paid to semiconductor devices having one-dimensional channels. The semiconductor device having the one-dimensional channel has a problem of what is called Anderson localization which predicts that an electron wave function localizes if a localization potential, V.sub.0, is larger than a certain limit value. However, the semiconductor device can be operated under the condition of V.sub.0 &lt;kT (where k is Boltzmann's constant and T is the absolute temperature).
In the one-dimensional channel, since the phase space of the final state is limited, electron scattering probability is extremely small and a remarkable increase in electron mobility .mu. is expected. Therefore, trials to form a one-dimensional channel have conventionally been performed. As one of the trials, there is a method of forming a one-dimensional channel by a lithography using an electron beam or the like. According to the method, although a one-dimensional channel having a width of about 500 .ANG. can be formed, it is difficult in the present situation to form a one-dimensional channel having a width less than 200 .ANG.. On the other hand, if it is tried to form a number of one-dimensional channels so as to be adjacent to one another by the above method, there are drawbacks such that the width is widened and the interval cannot be narrowed because of what is called a proximity effect. Further, according to the above method, there is also a drawback such that a damage is easily caused when a one-dimensional channel is formed by reactive ion etching (RIE) or the like.
FIG. 4 shows a conventional one-dimensional channel structure. As shown in FIG. 4, in the example an Al.sub.x Ga.sub.1-x As layer 202, an aluminum arsenide (AlAs) layer 203, a gallium arsenide (GaAs) layer 204, and an AlAs layer 203 are sequentially formed on a semiinsulating GaAs substrate 201. A V-groove 205 is formed in the Al.sub.x Ga.sub.1-x As layer 202, the AlAs layer 203, and the GaAs layer 204. A gate electrode 206 is formed on the V-groove 205. In the example, one-dimensional electrons are formed in the GaAs layer 204 at the interface with the gate electrode 206 and serve as a one-dimensional channel.
On the other hand, in a conventional one-dimensional channel structure shown in FIG. 5, the GaAs layer 204 and Al.sub.x Ga.sub.1-x As layer 202 are alternately grown on the semiinsulating GaAs substrate 201, the GaAs layer 204 and Al.sub.x Ga.sub.1-x As layer 202 are patterned by etching, and thereafter, an Al.sub.x Ga.sub.1-x As layer 207 is grown on the side surfaces of the layers 204 and 202 and the gate electrode 206 is formed beside the Al.sub.1-x As layer 207. In the example, a one-dimensional channel is formed by the one-dimensional electrons which are formed in the GaAs layer 204 at the hetero-interface between the Al.sub.x Ga.sub.1-x As layer 207 and the GaAs layer 204.
Further, a one-dimensional channel structure as shown in FIG. 6 is also known (Appl. Phys. Lett., 41(7), 635 (1982)). As shown in FIG. 6, in the example, the Al.sub.x Ga.sub.1-x As (x=0.25) layer 202 and the GaAs layer 204 are alternately grown on the whole surface of the semiinsulating GaAs substrate 201. A mesa structure having a triangular cross-section is formed by processing the GaAs layer 204 and Al.sub.x Ga.sub.1-x As layer 202 by using a photolithography and a chemical etching. After that, a semiinsulating Al.sub.x Ga.sub.1-x As (x=0.31) layer 208 is formed on the whole surface by a molecular beam epitaxy (MBE) process. In the example, a one-dimensional channel is formed in a quantum well fine line comprising the GaAs layer 204 which is surrounded by the Al.sub.x Ga.sub.1-x As layers 202 and 208 as barrier layers.
However, in the conventional one-dimensional channel structures shown in FIGS. 4, 5 and 6 since the surface of the GaAs layer 204 in which the one-dimensional electrons are formed is subjected to the atmosphere during the manufacturing process, there is a drawback such that the characteristics of the surface deteriorate, so that the characteristics of the one-dimensional channel deteriorate, too. Further, in the example shown in FIG. 6, there is also a drawback such that it is not always easy to form the mesa structure having the triangular cross sectional shape.
On the other hand, historically, development of an ultrahigh speed device has passed from a bulk device using a three-dimensional electron running layer such as the Schottky-gated field effect transistor (MESFET), junction-gate FET (JFET), or the like and a device using a two-dimensional electron running layer such as a high electron mobility transistor (HEMT) or the like to a one-dimensional channel device although not completed yet.
Although the above three-dimensional electron device and two-dimentional electron device have already been put into practical use, in those devices, phonons exert a definite influence on the operation of the device, causing the characteristics to be deteriorated. On the other hand, in the present situation, realization of a practical one-dimensional device is fairly difficult at a stage in which fluctuations of the current-voltage (I-V) characteristics or the like are measured. In addition, in the one-dimensional channel device, since a quantum effect appears only at a very low temperature because of the scattering by the phonons, it is considered that such a point will become a problem in future when the one-dimensional channel device is put into practical use.